A slide bearing for supporting a crank shaft is used as a slide member in applications such as engines, etc. for automobiles. The slide member lubricates in the presence of a lubricant. Oil is used as a typical lubricant. In such type of slide member, a diamond-like-carbon (DLC) layer is typically provided over a bearing alloy layer backed by metal. The bearing alloy layer serves as a substrate and is typically formed of materials such as a copper alloy. The structure provided with the DLC layer exhibits improved wear resistance.
For example, Japanese Patent Publication No. 2006-8853 A discloses a first slide member which may also be hereinafter referred to as a first conventional example. The first slide member is formed of a substrate and a hard carbon coating provided over the substrate. The hard carbon coating is formed of a laminate of a hard layer and a soft layer. The surface of the hard carbon coating is polished so that the hard layer in the lower layer of the laminate is exposed to provide a smooth sliding surface. Japanese Patent Publication No. 2009-13192 A discloses a second slide member which may also be hereinafter referred to as a second conventional example. The second slide member is formed of a substrate, a carbon nitride film (a-CNx film) provided over the substrate, and an amorphous carbon coating film provided over the carbon nitride film.
A slide member or a bearing for example provided with a DLC layer on its surface worked well without being damaged in a conventional engine environment. Nowadays, however, engines are required to provide further improved performance and functionalities. Especially in an environment where starting and stopping of the engine is repeated on a frequent basis such as in a hybrid automobile application, the slide member is exposed to increasingly severe conditions. Thus, further improvement is required in the wear properties of the slide member. For improvement in the wear properties of the slide member, it is important to reduce the friction coefficient in the boundary lubrication regime and accelerate the transition to the mixed lubrication regime.
The first slide member attempts to reduce the friction coefficient by smoothening its surface but does not allow oil to be drawn easily to the slide surface, making it difficult for an oil film to form. Thus, the first slide member exhibited high friction coefficient in the boundary lubrication regime. The second slide member achieves reduced friction coefficient in the boundary lubrication regime but fails to conform with the counter element since the conforming layer is too thin and thus, is not able to form an oil film. It is thus not possible to accelerate the transition to the mixed lubrication regime. The second slide member requires a nitrogen atmosphere and therefore is not intended for application to a slide bearing used with oil.